1. Field of the Invention
The present invention relates to a technique for making integrated circuits having improved fusible conductive links.
2. Description of the Prior Art
Integrated circuits frequently include fusible conductive links ("fuses") that may be rendered non-conductive (i.e., "blown") by the application of laser energy. For example, in dynamic or static memory chips, defective memory cells may be replaced by blowing the fuses associated with the defective cells, and activating a spare row or column of cells, which may also be accomplished by blowing fuses. Logic circuits may also be repaired or reconfigured by blowing fuses. For example, it is known to initially fabricate a generic logic chip having a large number of interconnected logic gates. Then, in a final processing step, the chip is customized to perform a desired logic function by disconnecting the unnecessary logic elements by blowing the fuses that connect them to the desired circuitry. Still other applications of laser-blown fuses are possible.
The reliability of blowing the fuses is significant, especially when a large number of fuses must be successfully blown for an integrated circuit to function properly. One limiting factor in successful fuse blowing is the minimum laser energy required to blow the fuses. That is, if the laser energy is not high enough, then some of the fuses that are to be blown will not in fact be rendered non-conductive. Such process variables as the thickness or width of the fuse, and the variations in the laser energy from one shot to the next, can affect the reliability of fuse blowing. On the other hand, the debris generated by successfully blowing the link can itself lead to reliability problems. That is, if the conductive link material (typically doped polysilicon, metal silicide, or a metal) is re-deposited on the chip after the fuse is blown, then it may land in an area that could cause reliability concerns. For example, the blown link material may re-deposit so as to short together two conductors on the same or different levels of the chip.
One technique for improving the reliability of blowing fuses with radiant (e.g., laser) energy is described in co-assigned U.S. patent application Ser. No. 07/084531 now U.S. Pat. No. 4,853,758. As taught therein, the dielectric material covering a fuse may be partially etched away, reducing its thickness over a fuse that is formed in a lower conductor level. This allows the fuse to blow more easily (that is, at a lower energy level) than if the full dielectric thickness were retained.